1. Field of the Invention
The present invention relates to a method and apparatus for forming a deposited film in which a thin film is formed on a substrate under a reduced pressure condition, and relates to, for example, a method and apparatus for forming a deposited film which are used for mass production of photovoltaic devices using amorphous silicon (amorphous semiconductor) or a non-single-crystal semiconductor such as microcrystalline silicon by plasma CVD or the like.
2. Related Background Art
Up to now, as regards photovoltaic devices used for solar cells and the like, attention has been paid on an amorphous material typified by amorphous silicon (a-Si:H) and a non-single-crystal semiconductor material such as microcrystalline silicon for the reasons that they are inexpensive, that they can be increased in size and reduced in thickness, that degrees of freedom thereof for composition are high, and that the electrical and optical characteristics thereof can be controlled in a wide range, and other reasons. In manufacturing the above-mentioned devices, an apparatus for forming a deposited film in which a thin film is formed by plasma CVD under a reduced pressure condition has been widely used in general and adopted in industries.
As regards the solar cells, the following points are essentially important: that a photoelectric conversion efficiency thereof is sufficiently high; it has excellent stability in its characteristic; and it can be manufactured through mass-production. Thus, it is required in manufacturing of the solar cells using a non-single-crystal semiconductor layer or the like that improvements be attempted in electrical, optical, photoconductive, and mechanical characteristics, fatigue characteristics to repetitive usage, and resistance to a usage environment of the solar cells to be manufactured. Also, it is required that the photovoltaic devices having a larger area and a semiconductor layer, which constitutes a solar cell, with a more uniform thickness and film quality are manufactured in mass-production at high speed by using a reproducible method.
The semiconductor layer, which serves as an important component in the solar cell, includes a semiconductor junction such as what is called a pn junction or a pin junction. In the case of using a thin film semiconductor such as a-Si, silane (SiH4) as a starting gas, which contains an element serving as a dopant such as phosphine (PH3) or diborane (B2H6), and the like are mixed to generate glow discharge so that the starting gas is decomposed, whereby the decomposed materials are attached and grown on a heated solid surface (plasma CVD). As a result, a semiconductor layer of desired conductivity type is obtained.
As described above, it has been known that semiconductor layers of desired conductivity type are sequentially stacked on a desired substrate, so that the layers can be easily formed to a semiconductor junction. In view of this, as a manufacturing method for a solar cell obtained by stacking non-single-crystal semiconductor layers, a method has been proposed in which: independent vessels for forming respective semiconductor layers are provided; the semiconductor layers are sequentially formed in the respective vessels; and resultingly, a stack of the semiconductor layers having a desired semiconductor junction is formed.
For example, the following method has been generally known in which: the independent vessels for forming semiconductor layers are coupled with each other through gate valves in a load lock system; and a substrate is moved to the vessels sequentially to form the respective semiconductor layers.
Further, as a method that remarkably improves mass-productivity, a continuous plasma CVD apparatus, which adopts a roll to roll method, is disclosed in U.S. Pat. No. 4,400,409. In this apparatus, a flexible substrate having a desired width and sufficient length is continuously conveyed through a conveying path provided in a plurality of glow discharge regions for performing glow discharge and forming semiconductor layers, and the semiconductor layers of required conductivity type are sequentially deposited on the substrate in the respective glow discharge regions, whereby a device having the plurality of semiconductor layers including a semiconductor junction is continuously manufactured.
However, such a mass-production apparatus for photovoltaic devices had a problem that generation of plasma over a large area for a long time gradually causes deposition of a film on an electrode surface, which varies a condition on electric power input. This means that the increase in thickness of the deposited film on the electrode surface changes an impedance of the electrode, which decreases a film formation speed.
As regards the above problem, Japanese Patent Application Laid-Open No. H11-233443 discloses a film forming method characterized in that input electric power is adjusted such that a self-bias voltage (Vdc) which is generated at a high-frequency electrode has a constant value. This method is effective in the initial period of film formation or in the case where the self-bias voltage has a small absolute value.
However, at the film formation or in the mass-production apparatus, which requires higher power application for raising a substrate processing speed or requires a long time process, control is difficult to be performed because an absolute value and a change amount of the self-bias voltage become large. This leads to the occurrence of the lowering in characteristics which accompanies the change in the self-bias voltage and the lowering of the film formation speed. Especially, the reduction in film thickness, which accompanies the lowering of the film formation speed at the time of formation of an i-type semiconductor layer, causes the decrease of a short-circuit current (Isc) as a solar cell characteristic, which largely affects the lowering of a conversion efficiency.
As described above, in the case of a film formation process with high electric power or for a long time in the prior art, it has been unavoidable that the condition change due to the film deposition (adhesion) on the electrode cannot be controlled, and the characteristics are gradually degraded with the elapse of time. That is, there has been a problem that, in the case where semiconductor devices are manufactured for a long time in mass production, variation in characteristics of the devices occurs with the elapse of time.
Moreover, in the prior art, the maintenance of a part and an electrode, on which a film is deposited, within a vacuum film formation vessel has been experimentally performed after film formation for a constant time in consideration of peeling of the deposited film and particles. However, the exact maintenance timing has not been determined in view of keeping the characteristics. Therefore, the lowering in characteristics occurs before the next maintenance in some cases resulting in one of the causes of the decrease in productivity.
Japanese Patent Application Laid-Open No. H07-335558 discloses a CVD apparatus provided with means for comparing a self-bias voltage with a set voltage to send a cleaning starting signal. In this application, description is made of the case of a discharge concentration phenomenon and the occurrence of particles. However, the above-mentioned lowering in the film formation speed occurs independent of those matters. That is, the lowering in characteristics may occur before the discharge concentration phenomenon and the occurrence of particles each exceed an allowable range thereof. Thus, the exact maintenance timing has not been realized in view of keeping the characteristics.